Electronic – zener diode protection on input of op amp

Putting the zener diode across the lower resistor in the divider is always going to disturb the voltage divider if the zener conducts any current at all. And apparently the zener that you have chosen conducts a fair amount of current and is far from an ideal unit.

If I was designing a protection for the A/D inputs I would use one of two approaches.

1. Use the zener diode on the signal voltage source before the voltage divider.

2. Use an actual diode clamp on the A/D input made of a dual diode component such as a BAT54S. The upper diode points into a clamp current sink made of a shunt supply that is designed such that the clamp works at approximately the 3.3V level. Something like this:

Can someone please explain why it is 2.15v?

If you look at the data sheet for the 1N4728 you will see this: -

I've added the bits in red i.e. I've taken the external zener voltage of 3.3 volts and subtracted from it the internal volt drop due to the impedance at the test current specified. Immediately you can see that a more realistic figure for the zener voltage at low currents is more like 2.5 volts. Look at this curve to see how poor the 1N4728 is at low currents compared to versions at higher voltages: -

At 10 mA, the graph suggests that the zener terminal voltage might only be 2 volts rising to over 3 volts at a current of about 50 mA. Compare this with the 1N4744 (15 volt zener) and you can see that it remains at or about 15 volts from 10 mA up to nearly 60 mA.

If you want a better solution for non-high powers you could consider the TL431 precision programmable shunt voltage reference: -